Flame resistant fibers prepared from a raw material of organic fibers such as polyacrylonitrile-based fibers are utilized as the disaster prevention heat insulating material for airplanes and the like and the material for brake discs because of their excellent flame resistance, flame retardancy, abrasion resistance and corrosion resistance, and because they have drape property and property for spinning that cannot be exhibited by flame resistant inorganic fibers, they are used also as the material of sputter sheets which protect a human body from high-temperature iron powder or welding spark generated in welding operation or the like. Further, such flame resistant fibers have been broadly used as heat insulating material, and as a material to be replaced with asbestos which has been regulated from its harmful affection to human bodies, and their demand is increased.
Further, the above-described flame resistant fibers are important as an intermediate raw material of carbon fibers. Carbon fibers are utilized for various uses because they have various excellent properties such as mechanical properties, light weight property and the like. As the uses of carbon fibers, for example, materials used for space/aero structures such as airplanes and artificial satellites, materials used for sports industrial goods such as golf shafts, fishing rods and tire wheels for bicycles, and materials used for architectural structures such as bridge girder reinforcing members and wind mills, can be exemplified. Furthermore, utilization of carbon fibers is increasing also in transportation and carriage machine uses such as automobiles, ships and trains. Further, because carbon fibers have a high conductivity, application to electronic parts such as chassis for personal computers is beginning. It is considered that in the future demand for carbon fibers will be further increasing, and stable and mass supply thereof is strongly desired.
Carbon fiber can be obtained by spinning and yarn producing a polymer solution prepared by dissolving mainly polyacrylonitrile (hereinafter, also abbreviated as “PAN”) in a solvent to induce the polymer into a PAN-based fiber, and burning it at a high temperature in an inert atmosphere. When the PAN-based fiber is employed as a precursor fiber of a carbon fiber, a heat resistant fiber or a flame resistant fiber, it passes through a process of gas-phase stabilization (cyclization reaction and oxidation reaction of PAN) which heats the PAN-based fiber in air at a high temperature such as 200 to 300° C. However, because an exothermic reaction progresses in the stabilization process, heat removal is required when a large amount of PAN-based fibers are stabilized. Therefore, for temperature control, a long-time treatment is required, and it is necessary to restrict the fineness of the PAN-based precursor fiber to a small fineness of a specified value or less to finish the gas-phase stabilization in a desired period of time. Thus, the known stabilization process cannot be said to be a sufficiently efficient process.
With respect to such a problem, in WO 2005/080448, JP-A-2007-31564, WO 2007/018136, JP-A-2008-095257 and JP-A-2009-197358, although shortening of time for the temperature control is succeeded by reacting an amine-based compound and an oxidant and performing liquid-phase stabilization to expedite the stabilization in a liquid, the spinning property (yarn producing property) and the flame resistance are not sufficient. Further, in JP-A-2009-91397, although the spinning property is improved by a metal hydroxide, the flame resistance is still not sufficient and, in addition, the properties when made into carbon fibers tend to be reduced by addition of the metal hydroxide. Furthermore, in JP-A-2009-149712, although an amino-sulfonic acid group is added to improve the spinning property, there is a problem in the flame resistance.
Accordingly, paying attention to the problems in the above-described conventional technologies, it could be helpful to provide a flame resistant PAN-based polymer capable of improving both yarn producing property and flame resistance, a polymer solution using the polymer, a flame resistant fiber using the polymer solution, a carbon fiber using the flame resistant fiber, and methods of producing the same.